Short of using other FString functions to sanitize the arguments ( e.g AppendInt(1) etc), you could look at modifying your engine to revert the offending change ( commit 6ed576a might be a good place to start investigating) or port the previous FString::Printf functionality from 5.5 into a plugin to be used under a different class name ( e.g MyFString::Printf and perform a sweeping text replacement where needed ), keeping in mind that the change to remove the insecure formatting arguments was made for a reason.